Production method and device for hologram

ABSTRACT

A method and apparatus for producing a hologram using a two-beam laser interference exposure process, comprising the steps of using as a light source a femtosecond laser having a pulse width of 900-10 femtoseconds and a peak output of 1 GW or more and capable of generating a pulse beam at or close to the Fourier transform limit, dividing the pulse beam from the laser into two by a beam splitter, controlling the two beams temporally through an optical delay circuit and spatially using plane and concave mirrors each having a slightly rotatable reflection surface to converge the beams on a surface of or within a substrate for recording a hologram at an energy density of 100 GW/cm 2  or more with keeping each polarization plane of the two beams in parallel so as to match the converged spot of the two beams temporally and spatially, whereby a hologram is recorded irreversibly on the substrate formed of a transparent material, semiconductor material or metallic material.

TECHNICAL FIELD

The present invention relates to the technology of holograms, and moreparticularly to a method and apparatus for efficiently producing ahologram having excellent embedability with a microscopic surface areaand thickness thereof, high diffraction efficiency and thereforeenhanced applicability to various recording substrates. The presentinvention also relates to a product incorporating such a hologram.

BACKGROUND ART

High energy density no fewer than 1 TW (10¹²W)cm³ may be obtained by afemtosecond laser. When a light having such a high energy density isirradiated onto a material, high-density electrons will be excited in ashort time period in the irradiated material. The energy of the excitedelectrons is converted into the vibrational energy of ions in thematerial within one nanosecond. Once the vibration energy densityexceeds a given threshold, the ions break away from the material,resulting in an abrasion of the material. The abrasion caused in thematerial generates microscopic holes, and thereby the effectiverefractive index of the material is locally varied. This phenomenon isreferred to as “micro-abrasion”. In this connection, when the vibrationenergy density is slightly lower than the threshold at which adestruction or abrasion is caused in the material, the material will notgo far enough to be destroyed but will cause a variation in therefractive index of the material in connection with a variation orstructural change in the atomic arrangement of the material.

It has been known to irradiate a high peak energy femtosecond laser beamto be converged at a spot having a small area onto a transparent crystalmaterial, such as silica glass, BK7 optical glass, plastic (acrylic),quartz crystal, or sapphire, to cause an abrasion in the material so asto create fine holes, or to form micropores within the material througha nonlinear refractive index effect, or to vary the refractive index ofthe material through the structural change of atomic arrangement in thematerial.

For example, E. N. Glezer and E. Hazur: Appl. Phys. Lett. 71,882,(1997), and K. Miura, J. Qie, H. Inoue, T. Mitsuya and K. Hirano: Appi.Phys. Lett. 71,3329, (1997) reports that an optical waveguide may beformed by increasing the refractive index at an arbitrary location in anamorphous material such as silica glass. Japanese Patent Laid-OpenPublication No. Hei 11-267861 discloses a method for forming a markingin a glass material. It has also been known to produce a diffractiongrating by forming a number of spots in a regular arrangement using adevice for irradiating a femtosecond laser beam onto a transparentmaterial.

However, the application of this production method of diffractiongratings to actual elements and apparatuses involves unacceptableinsufficiency. Further, limited few materials may vary the refractiveindex therewithin. In particular, as to a diamond crystal, any variationof the refractive index has not been achieved by this method.

The practical application of a titanium-sapphire laser has opened a wayto obtain a femtosecond laser beam having a high coherence. Heretofore,it has been reported that when a femtosecond laser beam was irradiatedonto a thin-film material formed of diamond or the like, a ripplepattern and/or a so-called Newton ring phenomenon caused likely by pulseinterference were recorded in the material (A. M. Ozkan et al; Appl.Phys. Lett. 75,3716, (1999)), and this has suggested the coherence ofthe femtosecond laser beams. However, the reason for generating such amicrostructure has not been clarified. Further, it has not beenpositively attempt to take advantage of the coherence of thetitanium-sapphire laser.

A hologram has been conventionally produced through a two-beam exposureoptical system by use of a gas laser output a high coherent continuousbeam, and a recording substrate formed of a photosensitive organicsubstance or inorganic compound. However, the low energy density of sucha gas laser has led to unmercifully long recording time and has forcedto limitedly use a recording substrate having high photosensitivity.While a pulse laser, such as a ruby laser, has been used to cut down therecording time, it is indispensably required to combinationally use thephotosensitive material as the recording matrix or substrate. Inaddition, it has been difficult to produce an embedded type hologram ora microhologram having a surface area of about 100 μm diameter or less.

DISCLOSURE OF INVENTION

Means for Solving the Problem

Heretofore, no report on a development of the two-beam exposureapparatus has been made, partially because it has been not clear if thecoherence of the conventional femtosecond laser beam could be maintainedsufficiently to enable the hologram recording. For example, a pulse beamhaving a pulse width of 100 femtoseconds is a short duration equivalentto a distance of only 30 μm, and its converged spot size is necessarilyarranged in about 100 μm diameter in order to provide a high energydensity. Further, the coherence of the high-density pulse can bedegraded due to a nonlinear optical effect of a recording substrateduring the propagation of the pulse through the substrate.

In view of the above conditions, the present invention provides a newlydeveloped two-beam hologram exposure process in stead of theconventional laser beam irradiation process using the photosensitivematerial, to achieve a method capable of recording a hologram on arecording substrate essentially having no photosensitivity which isformed of a transparent organic or inorganic material, semiconductormaterial or metallic material, by use of a pair of pulse beams branchedfrom a single pulse beam.

More specifically, according to the present invention, there is provideda method for producing a hologram using a two-beam laser interferenceexposure process comprising the steps of using as a light source afemtosecond laser having a pulse width of 900-10 femtoseconds and a peakoutput of 1 GW or more and capable of generating a pulse beam at orclose to the Fourier transform limit, dividing the pulse beam from thelaser into two by a beam splitter, controlling the two beams temporallythrough an optical delay circuit and spatially using both a mirrorhaving a planar reflection surface (hereinafter referred to as “planemirror”) rotatable slightly or finely and a mirror having a concavereflection surface (hereinafter referred to as “concave mirror”)rotatable slightly or finely to converge the beams on a surface of orwithin a substrate for recording a hologram at an energy density of 100GW/cm² or more with keeping each polarization plane of the two beams inparallel so as to match the converged spot of the two beams temporallyand spatially, whereby a hologram is recorded irreversibly on thesubstrate formed of a transparent material, semiconductor material ormetallic material based on a variation in the configuration of thesubstrate and/or a variation in the refractive index of the substrate inconnection with an abrasion of the substrate or a structural change inthe atomic arrangement of the substrate caused by the high densityenergy irradiation.

Preferably, the light source includes a femtosecond laser having a pulsewidth of 500-50 femtoseconds and a peak output of 10 GW or more and,more preferably, capable of generating a pulse beam close to the Fouriertransform limit. Preferably, the controlled beams are converged at anenergy density of 1 TW/cm² or more. For example, given that therefractive index of the substrate is 1.5, a pulse width of 100femtoseconds corresponds to a spatial distance of 20 μm and therebyprovides a hologram having a total thickness of 10 μm or less. Theposition or range of the depth of the hologram may be controlled bychanging at least one of optical path lengths of the two beams throughthe optical delay circuit, and the total thickness of the hologram maybe adjusted by changing the pulse time of each beam.

A titanium-sapphire laser beam pulse may be generated substantially atthe Fourier transform limit, and thereby has a significantly highcoherence. When such a coherent beam is split into two beams and thenthe two beams are temporally matched with each other again without anydegradation in coherence, an interference pattern having a clearcontrast between dark and bright regions will be provided if eachpolarization plane of the two beams is parallel. Thus, when a thresholddefined by the substrate is arranged between respective energy densitiesof the dark and bright regions, the interference pattern may be recordedas a relief pattern in the surface of the substrate or a variation inthe refractive index of the substrate caused in connection with anmicroabrasion of the substrate or a structural change in the atomicarrangement of the substrate.

In the method for producing a hologram according to the presentinvention, the following process may be employed.

Each position of the mirrors may be finely moved in the verticaldirection of each reflection surface of the mirrors and in the paralleland vertical directions of each incident beam, to vary each optical pathlength of the two beams so as to serve as the optical delay circuit.

A sum frequency from a nonlinear optical crystal, such as a BBO crystal,may be used to detect that the two femtosecond laser beams is matchedwith each other spatially and temporally. More specifically, when thecollision point between two beams exists within the crystal, a sumfrequency of the irradiated laser beams is generated by virtue of thenonlinear optical effect. From this point of view, the two beams may bematched spatially with each other within the BBO crystal and thenmatched temporally with each other by fine-adjusting the optical delaycircuit to maximize the intensity of the sum frequency.

When the nonlinear optical crystal, such as a BBO crystal, is used as amaterial providing the non-linear optical effect, each phase of the twobeams is necessary to be matched. This restricts an angle between thetwo beams, and this angle cannot be set large.

Air has a third order optical nonlinearity, and may be used as amaterial for detecting the spatial and temporal matching of thecollision position of the two beams. When using a femtosecond laserhaving a wavelength of 880 nm, a Third Harmonic Generation (THG) orThird Sum-Frequency Generation (TSG) (wavelength: 266 nm) is generatedbase on the third order nonlinear coefficient. The intensity of THG orTSG is in proportion to the square value of the intensity of theassociate beam. When two beams collide and interfere with each other,the resulting intensity of the bright region becomes four times greaterthan that of a single beam. Thus, the intensity of THG or TSG becomessixty four-fold. This allows the spatial and temporal matching to bedetected with an excellent sensitivity. In addition, using the thirdorder nonlinear property of air allows the restriction of the anglebetween two angles to be eliminated.

The energy density of the beam to be converged in the substrate may bearranged just below a threshold at which an abrasion is caused in thesubstrate, to modulate the refractive index of the substrate itself withkeeping the surface of the substrate in a flat or level configuration soas to form a surface type hologram.

In a silica glass, particular a silica glass including germanium, anoptically induced structural change is cased by irradiating a laser beamhaving a relatively low energy density, resulting in the volumetricshrinkage of about 3%. With this phenomenon, the energy density of thebeam to be converged onto the substrate may be arranged in the range ofa threshold at which an structural change is optically induced in thesubstrate to a threshold at which an abrasion is caused in thesubstrate, so as to form a surface relief type hologram based on thevolume change of the substrate in connection with the structural changeof the substrate induced by the laser beam. Further, in the substrate,there is a difference in an etching rate with an acid solution betweenthe structurally changed portion and another portion having nostructural change. Thus, the substrate having the surface relief typehologram recorded therein may be etched with an acid solution toincrease the depth of the surface relief and enhance the hologramdiffraction efficiency.

The incident position and incident angle of the two beams each incidentfrom the same direction into the substrate may be adjusted to form atransmission type hologram having an adjusted position in the depthdirection of the substrate and an adjusted surface area.

A part of the laser beam irradiated from the air onto the substrate isreflected by the surface of the substrate based on the difference in therefractive index between the air and the substrate. As a result, due toan interaction with the reflected light in addition to a light absorbingaction at the substrate surface, an additionally increased energy isabsorbed in the substrate, and thereby the substrate surface is subjectto abrasion. The reflection at the substrate surface may be cut out byimmersing the substrate into a solution having a refractive index closeto that of the substrate or by applying this solution onto thesubstrate, so as to cut out the abrasion to be cased in the substratesurface. The same effect may be obtained by coating an anti-reflectionfilm on the substrate.

A converging position of the two beams opposedly incident into thesubstrate and a size of a converged spot of the beams may be arrangedwithin the substrate by controlling the optical delay circuit and themirrors, so as to form an embedded reflection type hologram embeddedwithin the substrate and having an adjusted position in the depthdirection of the substrate and an adjusted surface area, wherein thelaser pulse time is further controlled to form an embedded reflectiontype hologram additionally having an adjusted total hologram thickness.In this case, the coherence of the femtosecond pulse is degraded duringthe propagation of the pulse through the substrate due to the nonlinearproperty of the substrate. Thus, a material having a lower nonlinearproperty may be used as the substrate or the energy density may bereduced as low as possible to provide an increase embedded depth of thehologram.

The in-depth position of the hologram within the substrate may be variedby controlling the converging position of the two beams opposedlyincident into the substrate by use of the optical delay circuit and acondensing lens, to embed a plurality of holograms in the depthdirection of the substrate, so as to provide a multiplex hologramrecording medium.

The substrate may include a transparent crystal or glass having noinversion symmetry property. In this case, the substrate may be adjustedin temperature and quasi-phase-matched with applying an electric fieldto form a domain inverted grating.

The substrate may also include a material transparent to visible lightselected from the group consisting of quartz, glass, sapphire, LiNBO₃,LiTaO₃, ZrO₂, CaF₂, diamond and acrylic resin, or a semiconductormaterial selected from the group consisting of silicon, Ge, GaAs, AIN,InP, GaN, ZnS, ZnSe, ZnO, SiC and any mixed crystal thereof. In thiscase, the formed hologram may be either one of a surface relief typehologram, a surface type hologram and a volume hologram.

The substrate may also include a metallic material selected from thegroup consisting of gold, silver, platinum, copper, nickel, chromium,aluminum, cadmium, tantalum and metal silicon, or a semiconductormaterial selected from the group consisting of silicon, Ge, GaAs, AIN,InP, GaN, ZnS, ZnSe, ZnO, SiC and any mixed crystal thereof. In thiscase, the formed hologram may be a surface relief type hologram.

Heretofore, the material for recording a hologram has been formed of aphotosensitive organic substance or inorganic compound, and therebyinvolved with many restrictions. In contrast, according to the method ofthe present invention, a hologram is irreversibly recorded based on theabrasion or structural change of the substrate caused by high-densityenergy, and thereby almost any materials may be used as the substrate.Further, once the hologram is irreversibly recorded, the recordedhologram may be kept stably for a long term, and never be lost unless itis heated up to a temperature to cause the structural change in theatomic arrangement of the substrate itself.

Preferably, the substrate is kept under vacuum pressure during theexposure. Keeping the substrate under vacuum pressure may preventparticles and fine-powders caused by the abrasion from attaching on thesubstrate surface, and thereby allow the substrate surface to be kept inan unpolluted state. This may also provide a flat surface of thesubstrate in the embedded type hologram.

Further, according to the present invention, there is provided ahologram formed irreversibly in a transparent material, a semiconductormaterial or a metal surface by means of the aforementioned method.

Further, according to the present invention, there is provided adistributed Bragg reflector (DBR) type or distributed feedback (DFB)type leaser comprising a diffraction grating composed of a hologramobtained from the production method of the present invention, a lasermedium selected from the group consisting of diamond, alumina, sapphire,and glass consisting of the composition including at least either one ofAl₂O₃ and SiO₂, wherein the laser generates a beam based on an inherentemission yielded from exciton effects and optical inter-bandtransitions, an emission yielded from inherent defects, or an emissionyielded from added impurities. More specifically, the DBR or DFB lasermay be produced by forming a waveguide having a high refractive index inthe matrix or substrate, and providing a light-emitting center ormechanism within the waveguide, followed by providing an embedded typehologram on each end of the waveguide.

Further, according to the present invention, there is provided anapparatus for producing a hologram, using the two-beam laserinterference exposure process in the aforementioned method. Thisapparatus comprises a light source of a femtosecond laser, a beamsplitter for dividing a pulse beam from the laser into two pulse beams,and an optical system including an optical delay circuit for temporallycontrolling a converging position of the pulse beams, plane and concavemirrors for spatially controlling the position, and a mechanism forfinely rotating the mirrors. This apparatus may further include anaperture control element for shaping the laser beam into a Gaussianpulse to lower a threshold for forming the hologram.

The two-beam laser exposure apparatus is essentially required to have anoptical system capable of being controlled positionally in micron scale.As an apparatus having a high-precision position control performance tocope with this requirement, the present invention provides an opticalsystem including the optical delay circuit allowing a fine control andthe plane and concave mirrors, with the function capable of detectingthe converging position of the two beams, whereby the two beam may beconverged on or within a substrate for recording the two beams as ahologram, with matching the converged spot of the two beams temporallyand spatially.

The apparatus on the present invention employs a mirror opticalcomponent in terms of using the minimum number of transparent opticalcomponent. This allows any degradation of the coherence of the two beamsto be eliminated. Further, in order to improve the interference betweenthe two beams, each polarization plan of the two beams may be paralleledby adjusting the arrangement and number of the mirrors in each opticalpath.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram showing an optical system in a method andapparatus for producing a hologram using a two-beam laser exposureprocess according to the present invention.

FIG. 2 is a conceptual diagram showing a temporal control of aconverging position of two pulse beams B1 and B2.

FIG. 3 is a conceptual diagram showing a spatial control of theconverging position of the two pulse beams B1 and B2.

FIG. 4 is a conceptual diagram showing an incident angle to two laserbeams in an example 1.

FIG. 5 is an enlarged plane view showing a pattern of a diffractiongrating recorded in the example 1.

FIG. 6 is an enlarged plane view showing a diffraction pattern in aHe-Ne laser using the diffraction grating recorded in the example 1.

FIG. 7 is a photograph as a substitution for a drawing showing an AFMimage of a diffraction grating recorded in an example 3.

FIG. 8 is a conceptual drawing of a process of subjecting a hologram inthe example 3 to a chemical etching.

FIG. 9 is a photograph as a substitution for a drawing showing an AFMimage of the etched hologram in the example 3.

FIG. 10 is a conceptual drawing of a method in an example 4.

FIG. 11 is a conceptual drawing of a method in an example 5.

FIG. 12 is a photograph as a substitution for a drawing showing an AFMimage of a hologram in an example 7.

BEST MODE FOR CARRING OUT THE INVENTION

FIG. 1 is a conceptual diagram showing an optical system of a method andapparatus for producing a hologram using a two-beam laser interferenceexposure process of the present invention. A laser beam irradiated froma light source of a femtosecond laser is reflected by a plane mirror M1,and is then divided into a beam B1 and a beam B2 by a half mirror HF1used as a beam splitter. The beam B1 is reflected by a plane mirror M2and a concave mirror M3, and is then converged on a surface of or withina substrate S1. A plane mirror M3′ and a lens L1 having a thin thicknessmay be used as a substitute for the concave mirror M3. The beam B2 isreflected by a plane mirror M4 and a plane mirror M5. After reflected bya concave mirror M6, the beam B2 is converged on the surface of orwithin the substrate S1. A lens L2 and a plane mirror M6′ may be used asa substitute for the concave mirror M6.

An optical path shown by solid lines is used for producing atransmission type hologram. For producing a reflection type hologram, anoptical path shown by dotted bold lines is used. In this case, the beamreflected by the concave mirror 6 is converged within the substrate S1through a plane mirror M7 and a plane mirror M8. The lens L2 and theplane mirror M6′ may also be used as a substitute for the concave mirrorM6. As shown by a thin-dotted line, a hologram forming process may bemonitored by irradiating the substrate S1 with a He-Ne laser beamreflected by a plane mirror M9 and then acquiring the resultingreflected light.

The plain mirrors M4 and M5 serve as an optical delay circuit.Specifically, the plain mirrors M4 and M5 are finely moved at one-micronlevel using a micrometer caliper to adjust a difference in the opticalpath length between the beams B1 and B2 and thereby to make theconverged spot of the two beams match temporally with each other.Further, the concave mirrors M3, concave mirror M6 or plane mirror M8 isfinely rotated using a micrometer caliper to make a converging positionof the two beams match spatially with each other. In order to preventdegradation in the coherence of the beams, the lenses L1 and L2preferably have a fully reduced thickness and a long focal distance. Inthis arrangement, each polarization plane of the beams is parallel atthe substrate S1. Each optical parameter of a formed diffractiongrating, such as a fringe distance or a focal distance in the case ofproviding a lens function thereto, can be determined as in aconventional two-beam laser interference exposure process using acontinuous beam.

The substrate S1 is placed on an X-Y stage, and is finely moved using amicrometer caliper to record a hologram having a fine area at adesignated position of the substrate S1. In the production method of thepresent invention, one hologram may be recorded by a single pulse laserbeam. Thus, a plurality of holograms may be recorded in a multiplexed ormultilayered form by irradiating a plurality of pulses time-serially. Ifthe substrate is positionally fixed during the time interval of twopulses, produced holograms are spatially superposed with each other. Ifeach polarization plane of the laser beams is rotated at a given angle,each hologram to be superposedly formed will be rotated by the givenangle. Particularly, when the polarization plane of one pulse is rotatedat a 90-degree to that of the other pulse and these two pulses areirradiated time-serially, the resulting holograms will be superposed andshaped into a grid-like hologram. The same grid-like hologram may beformed by rotating the substrate at a 90-degree for one of the pulsesinstead of rotating the polarization plane.

Further, by moving the substrate in the X and Y directions, a hologrammay be formed extensively over the surface of the substrate. Since theexposure time is significantly short, the substrate may be continuouslymoved. In particular, when the substrate is moved during the timeinterval of two pulses by one half of a fringe distance of a hologramrecorded with a single pulse, a hologram having a substantially halffringe distance may be formed. In a femtosecond laser having awavelength of 800 nm, a minimum fringe distance of a transmission typehologram with a single pulse is 400 nm. Thus, a hologram obtained fromthe method of the present invention may have a reduced fringe distancedown to 200 nm. A femtosecond laser having a shorter wavelength may beadvantageously used to reduce the fringe distance.

A suitable laser includes a regenerative amplification titanium-sapphirelaser, which may be suitably arranged in an oscillation centerwavelength of about 800 nm, a pulse width of about 100 femtoseconds, anda pulse energy of about 1 mj/pulse equivalent to a peak output of about10 GW. Preferably, a converged spot size is about 100 μm diameterequivalent to a peak energy of about 100 TW/cm².

FIG. 2 is a conceptual diagram showing a temporal control of aconverging position of the beams B1 and B2. For example, 100femtoseconds corresponds to a distance in vacuum pressure of 30 μm. Thatis, the beam exists only over a length of 30 μm. Further, when the laserwavelength is 0.8 μm, a pulse beam of 100 femtoseconds includes only 40of peaks and valley. Thus, when the optical path difference between thebeams B1 and B2 is not arranged in 30 μm or less, the two beam pulseswill never be overlapped even if they are propagated through the sameoptical path. This means that the two beam pulses will never be matchedtemporally. The state when the beams B1 and B2 are not matchedtemporally is shown as (A) and (C) in FIG. 2, and the state when theyare temporally matched is shown as (A) and (B) in FIG. 2

FIG. 3 is a conceptual diagram showing a spatial control of theconverging position of the beams B1 and B2. If the beams B1 and B2 aretemporally matched as shown in (A) and (B) of FIG. 2 at a convergingpoint where the beam B1 spatially intersects with the beam B2 as shownin FIG. 3, the two beams interfere with each other. When an opticalenergy density of a bright region in an interference pattern generatedfrom the above interference exceeds a hologram recording threshold inthe substrate, the interference pattern is recorded as a refractiveindex modulation in the substrate.

In the production method of the present invention, by condensing beamsat the surface of the substrate, a surface relief type hologram having arelief on the surface thereof and a surface type hologram having arefractive index modulation in the substrate may be formed. Further, bycontrolling the condensing to provide an interference fringe within thesubstrate, an bedded type volume hologram may be formed.

EXAMPLE Example 1

Using the two-beam laser interference exposure optical system, atransmission type hologram was recorded in ambient air. A combination ofthe plane mirror M3′ and lens L1 and a combination of the plane mirrorM6′ and lens L2 were used. A single crystal of sapphire (10×10×1 mm) wasused as the substrate S1 for recording a hologram therein. A laser beamwas entered perpendicular to the “c plane” of the sapphire singlecrystal. The pulse energies of the beam B1 and beam B2 were arranged in0.7 mJ and 0.3 mJ, respectively, and thus the laser output was about 1mj/pulse. Using a singe pulse, a hologram was recorded by convergingeach of the beams with a spot size of about 10 μm diameter. Further, inorder to bring the laser beam into a Gaussian distribution through ashaping process, an aperture control element Al was inserted in theoptical path. As a result, the energy of the beams B1 and B2 necessaryfor recording a hologram could be reduced by 20%.

The beams B1 and B2 were exposed in two cases, i.e. at each angle θbetween the beams B1 and B2 of 10 degrees and 30 degrees as shown inFIG. 4, and grating fringe distances of 1.5 μm and 3 μm was obtainedfrom the two cases, respectively. The fringe distance d of the obtaineddiffraction grating could correspond to a value derived from thefollowing formula by using a laser wavelength λ=800 nm and therefractive index of an air which gives n=1;

λ=n·d·sin θ/2

It could also be confirmed that the resulting hologram was a surfacerelief type hologram from a measurement using an AFM. The area of theformed hologram was about 50 μm. Further, since the hologram may berecorded by a single pulse, the hologram in one sample could beextensively recorded within a pulse rate by moving the X-Y stagecontinuously. FIG. 6 shows a diffraction pattern 13 projected on ascreen 12 when a He-Ne laser (λ=633 nm) was irradiated on thediffraction grating 11 obtained from this example. A diffracted light ofa high order was observed in this diffraction pattern, and a first-orderdiffracted light was about 20%. Thus, it was proved that thisdiffraction grating could be applied to various diffraction opticalcomponents.

Example 2

Using the same two-beam laser interference exposure optical system asthat of Example 1, a surface relief type hologram was recorded in ametallic film. In order to bring the laser beam into a Gaussiandistribution through a shaping process, an aperture control element Alwas inserted in the optical path. A metallic film formed on a glassthrough a vacuum deposition process and having a thickness of about 250nm was used as the substrate for recording a hologram therein. After theshaping process, the pulse energies of the beams B1 and B2 were 0.13 mjand 0.07 mj, respectively. The angle θ between the two beams was 20degrees. The fringe distance of the obtained diffraction grating couldcorrespond to a value derived from the following formula by using alaser wavelength λ=800 nm and an air which gives n=1;

 λ=n·d·sin θ/2

Example 3

Using the same two-beam laser interference exposure optical system asthat of Example 1, a surface relief type hologram was recorded in asilica film. A SiO₂ thin film (film thickness: 114 nm) formed on a Sisingle crystal through a thermal oxidization was used as the substrate.The energy intensity of each of the beams B1 and B2 was 25 μj, and thebeams B1 and B2 were converged with a diameter of about 100 μm on thesurface. The angle between the two beams was 90 degrees, and acalculated fringe distance of the obtained diffraction grating was 580nm.

FIG. 7 shows an AFM image of the obtained diffraction grating. From thisAFM image, it could be confirmed that a surface relief type hologramhaving a groove depth of 2 to 3 nm and a groove distance of 580 nm wasformed. The groove depth was 2 to 3% of the film thickness of the silicaglass, and this value could correspond to a shrinkage percentage inconnection with an optically induced structural change in the silicaglass. FIG. 8 conceptually shows a process for subjecting the obtainedhologram H1 to a chemical etching with an acid liquid L to provide ahologram H2 having an extended groove depth. The etching was performedusing a solution including 1% of hydrofluoric acid for 5 minutes. FIG. 9shows an AFM image of the hologram after the etching. From this AFMimage, it could be confirmed that the groove depth was increased to 18to 20 nm, and a diffraction grating having a high aspect ratio wasobtained.

Example 4

FIG. 10 shows a concept of a method in this example. As shown in FIG.10, using the same two-beam laser interference exposure optical systemas that of Example 1, a hologram was recorded in a silica glass immersedin a solution. The solution was selected from either one of water,hydrogen fluoride solution, acetone, ethanol, methanol, hydrochloricacid solution and nitric acid solution. The energy intensity of each ofthe beams B1 and B2 was 400 μj, and the colliding position of the twopulses was arranged within the substrate. When the laser beams wereirradiated in ambient air under the same condition, certain damage wascaused on the surface of the substrate by an abrasion arising from theirradiation. However, when the substrate was immersed in the solution, adiffraction grating could be formed only within the substrate withkeeping the surface of the substrate flat. In particular, when using asolution including 1% of hydrofluoric acid, no processing distortion wasdetected in the silica glass.

Example 5

FIG. 11 shows a concept of a method in this example. As shown in FIG.11, a pulse of a titanium-sapphire laser (wavelength: 800 nm, pulsewidth: 100 femtoseconds, pulse rate: 10 Hz) was divided into two beamsB1 and B2, and the beams were collided in ambient air. The energyintensity of each of the beams was 0.75 mj. The angle between the beamB1 and B2 was varied in the range of 0 to 180 degrees. The mirrors M1and M2 and the lenses L1 and L2 were finely adjusted to spatially matchthe two beams, and then the two beams were temporally matched by usingthe optical delay circuit. Spectrum of the pulse after the collision wasexamined by using a spectrometer.

When the two beams was matched spatially and temporally, a THG wavehaving a significantly higher intensity than that in the contrary casewas observed. This proved that this phenomenon could be effectivelyutilized to detect the presence of the spatial and/or temporal matchingof the two femtosecond pulse beams. This detection process is alsoeffective to determine the time width of the femtosecond pulse.

Example 6

Using the same two-beam laser interference exposure optical system asthat of Example 1, an embedded type hologram was recorded in a diamond.In order to bring the laser beam into a Gaussian distribution through ashaping process, an aperture control element Al was inserted in theoptical path. A natural diamond (Type IIa, SAWN cut, optical absorptionend: 220 nm) was used as the substrate S1, and the size of the substratewas 3×3×0.5 mm. The pulse energies of the beams B1 and B2 were 0.14 mJand 0.06 mJ, respectively. The two beams were converged within thesubstrate S1 with a diameter of about 100 μm. When the angle θ betweenthe beams B1 and B2 was 10 degrees, an embedded type hologram could beformed at a position inward about 1 μm from the surface of the substrateS1 with a fringe distance of about 3 μm.

From the measurement using an AFM, it could be observed that the surfaceof the substrate was kept flat. It could also be confirmed that agraphite carbon was formed by the Raman scattering at the region havingthe formed diffraction grating. A diffraction efficiency of about 20%was obtained by irradiating a He-Ne laser. In view of the above results,it could be proved that this hologram could be applied to variousembedded type diffraction optical components. It could also be provedthat this hologram could be applied to an anti-fake hologram.

Example 7

With incorporating the two beams matching detect process in the two-beamlaser interference exposure optical system of Example 1, a hologram wasrecorded in a silica glass film formed by subjecting a Si substrate to athermal oxidization process. The pulse energy of each of the beams B1and B2 was 20 μJ, and the angle between the two beams was 158 degrees.The two beams were converged on the surface of the substrate S1 with adiameter of about 100 μm. A diffraction grating having a groove distanceof 430 nm could be confirmed from an AFM image shown in FIG. 12.

INDUSTRIAL APPLICABILITY

A hologram obtained from the production method of the present inventionis useful for diffraction gratings in the fields of optical informationcommunication and optical memory technologies. A specific applicationmay include optical elements using a surface relief type hologram, suchas an optical wavelength dividing element and optical polarizationelement, or any optical device using such an element; or opticalelements using a volume hologram, such as an embedded type opticalwavelength dividing element, optical polarization element and opticalwavelength output equalization element, or any optical device using suchan element.

The specific application may also include surface relief type hologramsor volume holograms for ornament, marking or anti-fake. Further, amultiplex hologram recording medium and a quasi-phase-matched (QPM)higher-harmonic wave generating element be exemplified, and any deviceusing such an element may be exemplified.

Furthermore, the specific application may include a distributed Braggreflector (DBR) type or distributed feedback (DFB) type leaser deviceusing as a polarization element a hologram obtained by the method of thepresent invention, by use of a laser medium including a compositionselected from the group consisting of diamond, sapphire, alumina, andglass consisting of the composition including at least either one ofAl₂O₃ and SiO₂, wherein the laser device generates a beam based on aninherent emission, or an emission yielded from inherent defects or addedimpurities.

What is claimed is:
 1. A method for producing a hologram using atwo-beam laser interference exposure process, said method comprising thesteps of: using as a light source a femtosecond laser having a pulsewidth of 900-10 femtoseconds and a peak output of 1 GW or more, andcapable of generating a pulse beam at or close to the Fourier transformlimit; dividing the pulse beam from said laser into two by a beamsplitter; controlling said two beams temporally through an optical delaycircuit and spatially using plane and concave mirrors each having afinely rotatable reflection surface; and converging said two beams on asurface of or within a substrate for recording a hologram at an energydensity of 100 GW/cm² or more with keeping each polarization plane ofsaid two beams in parallel so as to match the converged spot of said twobeams temporally and spatially, wherein a hologram is recordedirreversibly on said substrate formed of a transparent material,semiconductor material or metallic material, based on a relief in theconfiguration of the surface of said substrate and/or a variation in therefractive index of said substrate in connection with an abrasion ofsaid substrate or a structural change in the atomic arrangement of saidsubstrate caused by the high-density energy irradiation.
 2. A method forproducing a hologram using a two-beam laser interference exposureprocess as defined in claim 1, wherein each position of said mirrors isfinely moved in the vertical direction of each reflection surface ofsaid mirrors and in the parallel and vertical directions of eachincident beam to vary each optical path length of said two beams, so asto serve as said optical delay circuit.
 3. A method for producing ahologram using a two-beam laser interference exposure process as definedin claim 1, wherein said energy density is arranged just below athreshold at which an abrasion is caused in said substrate, to modulatethe refractive index of said substrate itself with keeping the surfaceof said substrate in a flat configuration, so as to form a surface typehologram.
 4. A method for producing a hologram using a two-beam laserinterference exposure process as defined in either one of claims 1 to 3,wherein said substrate is formed of a silica glass or a silica glassincluding germanium, wherein said energy density is arranged in therange of a threshold at which the structural change of said substrate isoptically induced to a threshold at which an abrasion is caused in saidsubstrate, so as to form a surface type hologram based on the volumechange of said substrate in connection with said optically inducedstructural change.
 5. A method for producing a hologram comprisingsubjecting a surface type hologram produced by the method as defined inany one claim of claims 1 to 3, to a chemical etching so as to increasethe depth of the relief of the surface thereof.
 6. A method forproducing a hologram using a two-beam laser interference exposureprocess as defined in claim 1, wherein the incident position andincident angle of the two beams each incident from the same directioninto said substrate are adjusted to form a transmission type hologramhaving an adjusted position in the depth direction of said substrate andan adjusted surface area.
 7. A method for producing a hologram using atwo-beam laser interference exposure process as defined in claim 1,wherein a converging position of the two beams opposedly incident intosaid substrate and a size of a converged spot of said two beams arearranged within said substrate by controlling said optical delay circuitand said mirrors, so as to form an embedded reflection type hologramembedded within said substrate and having an adjusted position in thedepth direction of said substrate and an adjusted surface area, whereinthe time width of the laser pulse is controlled to form an embeddedreflection type hologram additionally having an adjusted total hologramthickness.
 8. A method for producing a hologram using a two-beam laserinterference exposure process as defined in claim 1, wherein an in-depthposition of a hologram within said substrate is varied by controlling aconverging position of the two beam opposedly incident into saidsubstrate by use of said optical delay circuit and said mirrors, toembed a plurality of holograms in the depth direction of said substrate,so as to provide a multiplex hologram recording medium.
 9. A method forproducing a hologram using a two-beam laser interference exposureprocess as defined in claim 1, wherein said substrate is formed of atransparent crystal or a glass having no in version symmetry property,wherein said substrate is adjusted in temperature and isquasi-phase-matched with applying an electric field to form a domaininverted grating.
 10. A method for producing a hologram using a two-beamlaser interference exposure process as defined in claim 1, wherein saidsubstrate is formed of a material transparent to visible light, selectedfrom the group consisting of quartz, glass, sapphire, LiNBO₃, LiTaO₃,ZrO₂, CaF₂, diamond and acrylic resin, or a semiconductor materialselected from the group consisting of silicon, Ge, GaAs, AIN, InP, GaN,ZnS, ZnSe, ZnO, SiC and any mixed crystal thereof, wherein the formedhologram is either one of a surface relief type hologram, a surface typehologram and a volume hologram.
 11. A method for producing a hologramusing a two-beam laser interference exposure process as defined in claim1, wherein said substrate is formed of a metallic material selected fromthe group consisting of gold, silver, platinum, copper, nickel,chromium, aluminum, cadmium, tantalum, super-hard metal and metalsilicon, or a semiconductor material selected from the group consistingof silicon, Ge, GaAs, AIN, InP, GaN, ZnS, ZnSe, ZnO, SiC and any mixedcrystal thereof, wherein the formed hologram is a surface relief typehologram.
 12. A method for producing a hologram using a two-beam laserinterference exposure process as defined in claim 1, wherein saidsubstrate is kept under vacuum during the exposure.
 13. A method forproducing a hologram using a two-beam laser interference exposureprocess as defined in claim 1, which further includes the steps ofproducing a plurality of identical or different holograms by use of aplurality of laser pulses, and superposing the holograms produced byeach of said pulses.
 14. A method for producing a hologram using atwo-beam laser interference exposure process as defined in claim 1,wherein the polarization pane of each laser pulse is rotated, or saidsubstrate is rotated with fixing said the polarization pane.
 15. Amethod for producing a hologram using a two-beam laser interferenceexposure process as defined in claim 1, wherein said substrate is movedin parallel during the irradiation of each laser pulse.
 16. A method forproducing a hologram using a two-beam laser interference exposureprocess as defined in claim 1, which further including the step ofimmersing said substrate into a solution having a refractive index closeto that of said substrate or applying said solution onto the surface ofsaid substrate, to form an anti-reflection film on the surface of saidsubstrate.
 17. A hologram formed irreversibly on a transparentcomposition transparent to visible light, semiconductor material ormetal by use of the method as defined in any one claim of claims 1 to 3,and 6 to
 16. 18. A distributed Bragg mirror (DBR) type or distributedfeedback (DFB) type laser comprising: a diffraction grating composed ofthe hologram as defined in claim 17; and a laser medium selected fromthe group consisting of diamond, alumina, sapphire, and glass consistingof the composition including at least either one of Al₂O₃ and SiO₂,wherein said laser generates a beam based on an inherent emissionyielded from exciton effects and optical inter-band transitions, anemission yielded from inherent defects, or an emission yielded fromadded impurities.
 19. An apparatus for producing a hologram using thetwo-beam laser interference exposure process as defined in claim 1, saidapparatus comprising: a light source of a femtosecond laser; a beamsplitter for dividing a pulse beam from the laser into two pulse beams;and an optical system including an optical delay circuit for temporallycontrolling a converging position of the pulse beams, plane and concavemirrors for spatially controlling said position, and a mechanism forfinely rotating said mirrors.
 20. An apparatus for producing a hologramas defined in claim 19, which further includes an aperture controlelement for shaping the laser beam into a Gaussian pulse to lower athreshold on the formation of the hologram.
 21. An apparatus forproducing a hologram as defined in claim 19 or 20, which furtherincludes means for detecting the temporal and spatial matching of thetwo beams using a Third Harmonic Generation wave or Third Sum-FrequencyGeneration wave from a femtosecond laser beam based on the nonlinearproperty of air.